ok so i guess i fix one thing then another arises....

i drove the car today w/o any problems and then i went to dinner when i came back batt DEAD!!!! the car would not turn the lights and horn and all would but NO TURN i got a jumper box and the car started w/o any problems... anyone knows what it might be?
i put a multimeter on the battery when the car is running and im getting 11.25v i did the same thing on my VW and the battery reads 13.45 when the VW is running could it be my alternator? what i cant understand is why does the car run if the batt is dead.. something is not charging i guess.